1. Field of the Invention
The present invention relates to a liquid crystal electro-optical device which is superior in both electrical characteristics and contrast and which can provide bright and uniform display over the entire screen.
2. Description of the Related Art
A liquid crystal electro-optical device is constructed such that a liquid crystal material, which is generally an organic material, is interposed between a pair of substrates. Light traveling through the liquid crystal material is modulated by varying the intensity of an electric field developing between electrodes formed on the pair of substrates. Display is a result of this optical modulation.
Therefore, if a particular electrical signal is applied to an electrode, it can be displayed as a visually recognizable state. Further, a desired image can be formed by applying image data to a plurality of electrodes.
The above conventional optical modulation used in liquid crystal electro-optical devices is effected in the following manner. The orientation of liquid crystal molecules is changed to be parallel with or perpendicular to the substrates by changing the intensity of an electric field developing perpendicularly to the substrates.
In this case, in general, light is modulated by utilizing the optical anisotropy, which is one of the features of the liquid crystal material. Therefore, a polarizing plate is disposed in this device to linearly polarize incident light.
However, in the liquid crystal electro-optical device which operates in the above manner, although a normal display state can be obtained when it is viewed from the direction perpendicular to the display screen, it exhibits dark, unclear display when viewed obliquely. In the case of color display, oblique viewing causes display in an unintended color.
This phenomenon is explained as follows from the relationship between output light of the liquid crystal electro-optical device and orientation of liquid crystal molecules.
With a scheme in which liquid crystal molecules are oriented perpendicularly to the substrates, there is obtained for certain display an orientation state that the major-axes of liquid crystal molecules are arranged in the same direction. In this case, output light is observed from a surface perpendicular to the liquid crystal arrangement direction.
In this state, if the device is viewed from a direction somewhat deviated from the perpendicular direction, the line of observation is deviated as much from the direction of the major axes of liquid crystal molecules. This means that the output light greatly depends on the viewing direction. Thus, the viewing angle characteristic with respect to a viewer such that the performance deteriorates as the viewing direction deviates from the perpendicular direction.
To solve the above problem, there has been proposed an operation mode which is different from that in conventional liquid crystal electro-optical devices. In this operation mode, the optical characteristics are changed by rotating liquid crystal molecules only in a plane parallel with the substrates. This operation mode, which will be hereinafter called "IPS mode," is described in detail in Japanese Examined Patent Publication No. Sho. 63-21907.
In the IPS mode, an electric field is formed between a pair of electrodes formed on a single substrate and hence its main component is parallel with the substrate and a liquid crystal layer. The electric field causes liquid crystal molecules to rotate in a plane parallel with the substrate. Therefore, the IPS mode solves the above-mentioned viewing angle problem resulting from the vertical alignment of liquid crystal molecules in a certain aspect of operation.
However, a polarizing plate is indispensable in the IPS mode because light and shade display is attained by utilizing the birefringence due to the optical anisotropy of a liquid crystal material. The use of the polarizing plate is a factor of reducing the light transmittance because absorbs a considerable part of incident light.
Various types of liquid crystal electro-optical devices have been proposed which decrease the degree of reduction in light transmittance caused by the polarizing plate. Among those techniques is a guest-host mode which does not require any polarizing plate and which can utilize, without any alteration, the manufacturing techniques of conventional liquid crystal electro-optical devices. In particular, an operation mode invented by White and Taylor, which will be hereinafter called "WT mode," is typical of the guest-host mode.
In general, the guest-host mode is an operation mode which utilizes a dichroic dye added to a liquid crystal material. The term "guest-host mode" derives from the fact that the liquid crystal molecules and the dichroic dye are regarded as a host and a guest, respectively.
In the case of WT mode, a phase transition occurs, in which the cholesteric liquid crystal is transited to the nematic phase. This is caused by voltage non-application/application switching. Therefore, selection between light absorption and transmission is effected.
The guest-host mode that does not require any polarizing plate is disclosed in "Liquid Crystal Display," supervised by T. Ohkoshi, edited by the Institute of Television Engineers of Japan, pp. 84-94, and "Fundamentals of Liquid Crystals and their Application to Displays," M. Yoshino and M. Ozaki, Corona Publishing Co., Ltd., pp. 143-147.
However, in conventional guest-host mode liquid crystal electro-optical devices, the optical axes of liquid crystal molecules are equally arranged in the direction perpendicular to the substrate surface in a light transmission state (FIG. 5). As a result, the above-mentioned viewing angle problem occurs even in the guest-host type liquid crystal electro-optical devices. For example, even in a non-colored state with application of an electric field, the screen is colored at certain viewing angles.
As described above, although the liquid crystal electro-optical device that operates in the IPS mode has the advantage of a wide viewing angle, it is disadvantageous in that the brightness is low due to the use of a polarizing plate.
In contrast, the guest-host mode that does not require any polarizing plate has the advantage that it can output incident light without decreasing its intensity, that is, it can efficiently utilize light. However, like other conventional liquid crystal electro-optical devices, this mode has the problem of a strong dependence on the viewing angle.